1. Field of the Invention
The present invention relates to a gear cutting method and apparatus for generating teeth with use of a hob having a cutting edge portion made of a high-speed tool steel.
2. Description of the Prior Art
A conventional hobbing machine will be described below with reference to FIGS. 12 and 13, of which FIG. 12 illustrates an entire construction of the conventional hobbing machine and FIG. 13 illustrates the details of a cutting portion.
As shown in FIG. 12, a work 3 is supported on a hobbing machine 1 through a mounting means 2, and a hob 5 having a cutting edge portion made of a high-speed steel is supported by a hob head 4 so that it can be driven rotatively. As shown in FIG. 13, the mounting means 2 is supported by a table 6, and the work 3 is adapted to rotate with rotation of the table 6. A hob spindle 7 is attached to the hob head 4 so that it can be driven rotatively. The hob 5 is fixed to the hob spindle 7 so as to be rotated by operation of the hob spindle 7. In FIG. 13, numeral 8 denotes a nozzle for supplying a cutting oil 9 to the cutting portion.
For generating teeth on the work 3 by means of the hobbing machine 1, the work is loaded onto the table 6 through the mounting means 2 and the hob 5 is fixed to the hob spindle 7 in the hob head 4. Then, the table 6 is rotated to rotate the work 3 and likewise the hob spindle 7 is rotated to rotate the hob 5. In this rotating state of the hob 5 and the work 3, the hob is allowed to cut into the work, with the result that the outer periphery of the work is cut out by the cutting edge portion of the hob to generate teeth.
Since hobbing is a gear cutting method using a similar motion to the gear meshing motion, the number of revolutions of the hob 5 and the work 3 and the depth of cut of the hob are determines so as to generate predetermined teeth on the outer periphery of the work. During the gear cutting work, the cutting oil 9 is fed from the nozzle 8 to the cutting portion to lubricate and cool the cutting portion.
In order to reduce the machining cost in teeth generation by the hobbing machine 1, it is necessary that the machining be carried out in a short time. In the present situation, however, the peripheral speed (cutting speed) of the cutting edge portion of the hob 5 is 120 m/min or so at most due to the wear of the hob 5, for example, and thus a limit has been encountered in the rotational speed of the hob 5 and shortening of the machining time. This is presently an obstacle to the reduction of the machining cost.
Recently, a high-speed machining technique with a cutting speed of 200 to 300 m/min using a hob made of cemented carbide has been developed, and thus the generation of teeth by a hobbing machine has become more efficient. In the case of using a hob made of cemented carbide, a heat crack will develop if machining is performed under the supply of cutting oil, because cemented carbide is fragile. For this reason, in the case of using a hob made of cemented carbide, a dry cutting method in which machining is performed without the supply of cutting oil is mainly adopted. Since cemented carbide is much higher in both heat resistance and wear resistance than the high-speed steel, no problem occurs even if such a dry cutting is performed.
As mentioned above, the machining efficiency is improved by using a hob of cemented carbide and it may become possible to reduce the machining cost by the increased cutting speed. However, the hob of cemented carbide is so expensive that the total cost becomes extremely high even if the machining efficiency is improved. In addition, since cemented carbide is fragile, there is a fear that a sudden crack may occur. For this reason, the hob of cemented carbide is not presently in wide practical use.